Hyperpolarized carbon-13 magnetic resonance imaging (“MRI”) is an emerging technology, which seeks to enhance the existing capabilities of modern diagnostic MRI, with the capacity to monitor and visualize cellular metabolism in patients. The technique holds promise for probing pathological metabolism associated with various disease states including cancer and heart failure. A powerful feature of hyperpolarized carbon-13 MRI is the implicit registration of patient data obtained from conventional proton MRI and metabolic hyperpolarized carbon-13 exams, enabling multi-modal data fusion.
Because the hyperpolarized state is short lived, rapid acquisition schemes are necessary to permit both time-resolved and volumetric imaging. For hyperpolarized substrates, such as [1-13C]pyruvate, which give rise to sparse metabolic spectra, frequency-selective acquisition schemes can be leveraged to avoid the need for direct chemical shift encoding by resolving individual metabolite signals “up front” with spectral-spatial RF pulses. Once a single resonance has been excited, single-shot spatial encoding techniques can be employed to generate a 2D image within one repetition time (“TR”). When gating is unnecessary, echo-planar imaging offers benefits over spiral imaging due to its simpler image reconstruction and robustness to gradient delays.
In contrast to spectroscopic techniques, which directly encode both spatial and spectral content, echo-planar spatial encoding is very sensitive to variations in the local resonance frequency. Off-resonance magnetization can give rise to pixel shift artifacts along the blip encoding direction because of linear phase ramps in k-space that are caused by the off-resonance sources.
Static B0 field mapping methods represent the gold standard approach for correcting EPI distortion in conventional proton MRI; however, the true off-resonance map for hyperpolarized carbon-13 signals is unavailable before the hyperpolarized carbon-13 injection, and performing a field mapping acquisition wastes precious, non-renewable magnetization. Inhomogeneities induce distortion in the metabolic maps, compromising the implicit spatial registration with respect to the underlying anatomy.
Thus, there remains a need to provide a method for producing distortion free images from hyperpolarized carbon-13 acquisitions without the need for time consuming field mapping acquisitions that waste non-renewable magnetization. Such methods should advantageously be applicable to other imaging applications, including diffusion imaging and other time-resolved or volumetric imaging techniques.